1. Field of the Invention
The invention relates to composite structures comprised of reinforcing fibers and a thermosetting resin matrix and means for enhancing the bonding characteristics of the fiber and resin while improving the flexural strength and shear strengths of such composites.
2. Description of the Prior Art
Polyester base resins are derived from the reaction of a diol or glycol, such as propylene glycol, with a mixture of two different dicarboxylic acids. One of these acids is unsaturated and is customarily maleic or fumaric acid, and the other acid is usually either phthalic acid or isophthalic acid. The resulting reaction product is a relatively low molecular weight linear polyester having reactive partially unsaturated structure. The final resin curing process consists of a peroxide-catalyzed reaction with a reactive unsaturated monomer to generate a cross-linked thermoset structure.
Styrene is the most often used monomer in thermoset polyesters. Other monomers sometimes used include vinyl toluene, methyl acrylate, divinyl benzene, diallylphthalate and triallyl cyanurate. There are a large variety of peroxides useful in curing polyesters including, among many others, t-butyl perbenzoate, benzoyl peroxide, 2-butanone peroxide, t-butyl peroctoate and 1,1-bis(t-butyl peroxy)cyclohexane.
Reinforcing fibers known in the art as being useful for providing strength and durability to resin matrix materials include those of carbon, glass, silicon carbide, boron and quartz. The term fibers is used herein in the generic sense and includes fibers in filament, tow, staple yarn, roving, woven or chopped configuration. Likewise, the term carbon fibers is used generically, and includes both graphite fibers and amorphous carbon fibers.
Composite structures formed from polyester base resins and reinforcing fibers, especially of carbon, often exhibit low interlaminar composite shear strength, which is a measure of interfacial bonding between the fibers and the resin, because of relatively poor adhesion between the fibers and the matrix.
In the past, improved bonding has been attempted by coating the surfaces of carbon fibers or the like with various metals (e.g., tantalum), metal carbides (e.g., whiskers of silicon carbide) and nitrides. These coating processes are inherently expensive and it is difficult to control the thickness and homogeneity thereof. Further, these coatings do not enhance the flexural or shear strengths of the final composite structure.
Additionally, various surface treatments known in the art have been applied to carbon fibers or the like to improve their bonding characteristics. Such treatments, however, are conventionally oxidative or corrosive etches of various kinds, and include treatment with air, ozone, concentrated nitric acid, chromic-sulfuric acid, nitrogen dioxide and the like. Although these treatments do tend to improve composite shear strength, the reactants adversely affect the internal portions of the fibers and actually degrade other fiber and/or composite physical properties. Furthermore, surface treatments of this nature are expensive, relatively inconvenient to apply to the fibers, often pollute the atmosphere, and can be hazardous to the health of workers in the area where the treatment is taking place.